"If I had to pick any possible mistress it would be Brad's career. He worked really hard to get into leading man status," a source told US Weekly.

"Renée had to take a backseat. She travelled with him to make it work."

The insider went on to claim that Zellweger was more dedicated to the relationship, adding: "She's accomplished a lot of her professional goals. So Renée took some time out to be a great girlfriend and see if this is what it took to make a relationship work.